dm crypt: backport version 1.11.0 to get support for allow_discards

This change provides allow_discards, which was added in version 1.11.0 (Linux kernel 3.1). This enables android 4.3 built-in TRIM feature when using android encryption feature, thus greatly reducing lag and improving responsiveness.

I've been testing this change for 2 months on my only phone without issues. You can also see some successful reviews on CM's gerrit: http://review.cyanogenmod.org/#/q/owner:%22Ignacio+Larrain%22+status:open,n,z